Additive Manufacturing of Metallic Cellular Materials via Three-dimensional Printing

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ORIGINAL ARTICLE

Additive manufacturing of metallic cellular materials

via three-dimensional printing

Christopher B. Williams &Joe K. Cochran &

David W. Rosen

Received: 4 January 2010 /Accepted: 22 June 2010 /Published online: 17 July 2010# Springer-Verlag London Limited 2010

Abstract Cellular materials, metallic bodies with gaseousvoids interspersed throughout the solid body, are a

promising class of materials that offer high strength

accompanied by a relatively low mass. Recent research

has focused in the topological design of cellular materials in

order to satisfy multiple design objectives. Unfortunately,

these design advances have not been met with similar

advances in cellular material manufacturing as existing

techniques constrain a designer to a predetermined part

mesostructure, material type, and macrostructure. In an

effort to address these limitations, the authors have

developed a manufacturing process chain centered on an

augmented three-dimensional printing process. Specifically,metallic cellular materials are made by selectively printing

solvent into a bed of spray-dried metal oxide ceramic

powder. The resulting green part is then sintered in a

reducing atmosphere to chemically convert it to metal. Theresultant process has produced maraging steel cellular

artifacts featuring a 270-m wall thickness and angled

trusses and channels that are less than 1 mm in diameter.

Keywords Additive manufacturing . 3D printing .

Cellular materials . Designed mesostructure

1 Manufacturing parts of designed mesostructure

1.1 Parts of designed mesostructure

When modern man builds large load-bearing structures, he

uses dense solids; steel, concrete, glass. When nature does

the same, she generally uses cellular materials; wood, bone,

coral. There must be a reason for it [1]. The observations

of cellular materials found in the natural world have

directed more than 50 years of research towards manufac-

turing processes capable of producing metallic cellular

materials. These structures, which feature gaseous voids

interspersed throughout the solid body, are valued for

having high strength accompanied by a relatively low

density [2]. These materials can also offer large stiffness,

improved impact absorption, and thermal and acoustic

insulation to their applications [3].

Recent research has focused in designing the mesoscopic

topology (the geometric arrangement of solid phases and

voids within a material or product on the size range of 0.1

to 10 mm) of cellular materials in order to effectively

support and improve multiple design objectives of the

artifact [4, 5]. Example parts of designed mesostructure

C. B. Williams (*)

Department of Mechanical Engineering,

Virginia Polytechnic Institute and State University,

114F Randolph Hall,

Blacksburg, VA 24061, USA

e-mail: [email protected]

URL:http://www.me.vt.edu/dreams

J. K. Cochran

School of Materials Science and Engineering,

Georgia Institute of Technology,

Atlanta, GA, USA

D. W. Rosen

School of Mechanical Engineering,

Georgia Institute of Technology,

Atlanta, GA, USA

Int J Adv Manuf Technol (2011) 53:231239

DOI 10.1007/s00170-010-2812-2
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include an acetabular cup (Fig.1a) in which the porosity of

the truss structure has been designed to match the porosity

of the recipients bone so as to encourage bone growth

upon implantation [6] and a trussed robot arm (Fig.1b) that

has been optimized to minimize mass while meeting

strength and deflection constraints [7].

1.2 Cellular material manufacturing

Unfortunately, traditional cellular material manufacturing

processes prevent the realization of these design improve-

ments. Due to processing limitations, existing processes

limit cellular topology to either a random assortment of

voids (e.g., metal sponging and foaming processes such as

Hydro/Alcan/Combal, Alporas, Formgrip, Gasar, etc. [1,3])

or an ordered repetition of a unit cell (e.g., joining crimped

sheet metal into a corrugated form [8], bonding metal

textile screen meshes [9], sand/investment casting trussed

lattices [10], etc.[11]). In addition to limiting cellular

topology, the processes also constrain part macrostructure(most processes only offer planar geometry [12]) and

material selection [1]. While these processes are capable

of producing light-weight and strong cellular materials,

these limitations prevent a designer from tailoring part

mesostructure for specific design intent(s) [4,13].

There are several research efforts to address the

limitations of traditional cellular material manufacturing

via the use of additive manufacturing (AM) technologies to

create parts of designed mesostructure. Through their

additive, layer-based building process, AM technologies

(a.k.a., rapid prototyping, solid freeform fabrication, or

layered manufacturing) offer the utmost geometric freedom

in the design and manufacture of an artifact.

As previous efforts to use polymer-based AM to

indirectly create metal cellular structures through lost mold

and investment casting techniques resulted in porous parts

with limited cell sizes [14,15], recent research has focused

in creating metal parts with direct-metal AM techniques.Generally, the majority of these approaches are generally

not ideal for manufacturing cellular materials due to

limitations from poor resolution, poor surface finish, poor

material properties, limited material selection, and need for

support structures [13].

Ultrasonic consolidation [16] has been used to create

closed aluminum honeycomb panels; however, the technol-

ogy cannot build free-standing, unsupported, and angled

ribs and trusses, thus limiting its ability to create complex

cellular geometries [17]. Selective laser melting [18, 19],

direct-metal laser sintering [20], and electron beam melting

[21] have been successfully used to directly fabricatemetallic cellular materials with designed mesostructure.

While these technologies, which scan an energy source

(e.g., laser or electron beam spot) over a powder bed of

metal, are capable of creating fully dense parts with a small

feature size, they have inherent limitations:

& These processes are generally expensive (e.g., need for

a high-powered energy source) and have slow build

rates (e.g., vector scanning a small energy spot).

& The use of a high-powered energy source can introduce

residual stresses into the part, which arise from the high

thermal gradients present in the material during partfabrication [22]. This can lead to curling and/or warping

during the build; as such, support structures, which can

be difficult to remove from small cells, must be added

to the part geometry.

& Thermal gradients (and thus warping and residual stresses)

can be reduced by first preheating the powder bed with the

energy source. This technique lightly sinters the powder

bed before reapplying the energy source at an increased

power (and/or decreased scan speed) to fully melt the

powder to create the part [23]. However, pre-sintering the

powder increases its strength, thus making the loose

powder difficult to remove from the part; a feature that

might hamper the fabrication of cellular artifacts.

& Defects on bottom-facing surfaces and an overall poor

surface finish typically arise due to the surface tension

of the molten metal, which dominates at the small sizes

required to achieve good surface finish and creates the

potential for capillary instabilities [24].

& Problems arise when building over loose powder

(which is common when creating overhanging surfaces)

because the conductive heat transport is significantlyFig. 1 Parts of designed mesostructure: a acetabular cup [6] and b

trussed robot arm [7]

232 Int J Adv Manuf Technol (2011) 53:231239
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larger than when building over previously melted

powder [25]. This effect can cause delaminations,

buckling, and warping of cellular mesostructure [26].

In an effort to address the limitations of existing cellular

material manufacturing techniques, the authors have devel-

oped a process for the realization of metal parts with

designed mesostructure. Specifically the authors haveaugmented the three-dimensional printing process (3DP)

for the creation of green parts (formed from metal oxide

ceramic powders) that are suitable for conversion to metal

via thermal chemical post-processing.

2 Manufacturing process chain: three-dimensional

printing of metal oxide powders

The manufacturing process chain proposed by the authors is

composed of three steps, as presented in Fig. 2:

a. Material preparation. Fine metal oxide powders arespray-dried with a binder to form granules suitable for

processing with a 3DP machine.

b. Artifact creation via three-dimensional printing.Cellu-

lar ceramic green parts are fabricated using 3DP.

c. Post-processing. Following the removal of excess pow-

der, the metal oxide green part is sintered in a reducing

atmosphere thus chemically converting it to metal.

As the crux of the overall process is focused in the post-

processing stage, each phase of the process is described in

this section in reverse order.

2.1 Post-processing: chemically converting metal oxide

green parts to metal via reduction

In an effort to circumvent the limitations and difficulties (e.

g., warping, residual stresses, etc.) typically encountered

when directly fabricating metal parts via laser-based AM, the

authors take inspiration from the linear cellular alloy (LCA)

manufacturing process invented by the Georgia Tech Light-

weight Structures Group [27]. In this process, metal oxide

ceramic green artifacts are formed via extrusion and are then

sintered in a reducing atmosphere. The ceramic precursor is

chemically converted to metal, as the reducing agent

typically a gas (e.g., hydrogen or carbon monoxide)reactswith the oxygen of the green part and forms water vapor,

which is then removed from the system [28].

Cochran and coauthors have used this thermal/chemical

procedure to process a number of transition metal oxides

(Fe, Ni, Co, Cr, N Cu, Mo, W, Mn, and Nb), as well as

many engineering alloys (stainless steel, maraging steel,

Inconel, and Super Invar [29]) that are comparable to

conventionally processed counterparts [27]. The primary

requirement for this process is that the metal oxide must be

reducible at moderate temperatures (below the melting

points of the materials involved) with a partial pressure of

oxygen not lower than 1016 atm. This requirementexcludes some elements such as Ti and Al because they

are stable under these conditions; hence, they cannot be

introduced into the alloy as an oxide and must be added in

secondary processes.

Cellular materials featuring cell sizes in the range of 0.5

to 2.0 mm with web thicknesses of 50 to 300 m have been

created with the LCA process [27]. These small features are

accomplished, in part, by the shrinkage that is accompanied

with the reduction process (typically on the order of 30% to

70% by volume). This large shrinkage can cause cracks

and/or warping if not controlled carefully [28]; however, it

can be advantageous when fine geometric features are

desired that otherwise would be difficult or expensive to

fabricate [29].

Chemical reduction of metal oxide green parts to metal

has the potential to alleviate many of the limitations found

Step One Step

Three-Dimensional Printing

Two

Material Preparation

OxidePowders

BinderDrying

Spray Drying

Step Three

Sintering & H2Reduction

Finished Metal Part Direct Reduction

RollerPrint

Head

Powder Feed

Piston

Built

PistonBuilt Object

Binder

Supply

Fig. 2 Three-dimensional print-

ing of spray-dried metal oxide

powder followed by sintering

and reduction

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in direct-metal AM of cellular materials. The thermal/

chemical post-process provides a manner in which to create

metal artifacts without the application of thermal energy in

the forming stage of a manufacturing process chain, thus

avoiding difficulties with thermal transport phenomena

found in current direct-metal AM technologies (Sec-

tion 1.2). Furthermore, implementing this post-processing

technique is economically efficient, as the cost differentialbetween a metal oxide powder and its metal counterpart is

usually better than a 1-to-10 ratio [29]. Fine oxide powders

are readily available in a pure and stable form. Compared to

pure metal powders, metal oxides are safer as they are

neither carcinogenic nor explosive.

2.2 Artifact creation: three-dimensional printing

As the use of extrusion in the LCA process limits part

geometry to linear macrostructure and an ordered meso-

structure (that is constant throughout the extrusion), the

authors look to combine the material strengths of the post-process (Section 2.1) with the geometric freedom offered

by AM. Following a structured design process that featured

a conceptual design phase and a formal selection process

[30, 31], the authors determined that 3DP was the most

suitable AM technology for creating green cellular parts

composed of metal oxide ceramic material [32].

3DP features the selective printing of a binder over a bed

of powder via an array of inkjet nozzles [33] (Fig. 3). As

the binder enters the powder bed, it selectively joins

together powder particles to form printed primitives, which

stitch together to form a cross-sectional layer. A roller is

used to add a new layer of powder (at the desired thickness)

onto the previously printed layer. Excess powder from this

recoating process is caught into an overflow container for

reuse.

The authors chose 3DP as a method for creating metal

oxide green cellular parts for the following reasons:

& Speed: The parallel deposition of the multiple nozzles

enables the 3DP technology to deposit entire portions of

a layer in a single pass, thus dramatically increasing its

build speed.

& Cost: The two-dimensional patterning process imple-

mented by 3DP is inherently scalable [34]; unlike laser-

based systems, the quantity oftoolheads(i.e., printing

nozzles for 3DP) can be increased with very little

increase in overall cost. One can imagine creating an

array of print heads that would cover the entire width ofthe working area, such that only one linear stage is

needed to sweep along the area, and thus increasing the

deposition rate (and therefore reducing build time)

significantly.

& Resolution: The use of an inkjet printing nozzle to

pattern binder provides the 3DP technology the ability

to create parts with high resolution (minimum feature

size of ~0.1 mm), which is a crucial requirement for the

realization of cellular materials. The use of inkjet

printing technology also enables the creation of cross

sections that are characteristic of cellular materials. This

is not true to all AM patterning techniques; extrusionprocesses, for example, are unable to satisfactorily

deposit the small, discrete ellipses typically found in

the cross sections of trussed structures (Fig. 4b) due to

pores created by poor optimization of material flow,

filament/roller slippage, liquefier head motion, and

build/fill strategies [35].

& Complex geometry:Many AM technologies must create

support structures to facilitate the construction of

overhanging features (e.g., the trusses in Fig. 4a). Such

structures are not desirable when creating parts with

designed mesostructure because they would be very

difficult to remove from the parts small pores and

channels. The 3DP process eliminates the need for

specialized support structures, as the unpatterned

powder in the bed provides inherent support for the

complex geometry typical of cellular material. Unpat-

terned powder is easily removed from open cells via

careful use of compressed air and a vacuum nozzle. One

drawback of this approach, however, is that unpatterned

Powder Feed

Piston

Build

Piston

Roller Print

Head

Binder

SupplyBuilt Object

x

z

Fig. 3 Three-dimensional printing Fig. 4 Example a trussed material and its b cross section

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powder cannot be removed from closed cells and can be

troublesome to remove from very small channels.

& Green part density: By working with the powder in its

raw, loose form, 3DP avoids the solids loading and

rheology constraints found in the other ceramic AM

technologies that work with powder/binder suspensions

(e.g., aqueous [36] and hot-melt [37] direct inkjet

printing, extrusion [38], and stereolithography [39]).Compared to other ceramic AM processes (35 vol.%

solids in direct hot-melt inkjet printing [37], 40 vol.%

solids in extrusion [40]), 3DP has a relatively high

green part solids loading (powder beds with as high as

55 vol.% have been observed [41]).

2.3 Material preparation: spray-dried metal oxide powders

Fine powder particles are preferred over coarse particles in

ceramics processing because they have better sintering

characteristics, resulting in a finished part with a higherrelative density and better material properties. However,

one of the primary limitations with 3DP is its inability to

properly spread fine dry powders with particle size less than

20 m [42, 43]. The authors look to circumvent this

limitation by combining several fine particles into a larger

granule, which is more suitable for the 3DP recoating

process, via spray-drying.

Spray-drying is the process of spraying a slurry, composed

of fine powder particles (15 m) and a binder, into a warm-

drying medium to produce powder granules that are relatively

homogenous [44]. Spray-dried granules are nearly spherical

and typically on the order of 30 m in diameter; therefore,they flow very well and are easily recoated in the 3DP

process [45]. While the porous nature of spray-dried

powders (60 vol.%) is detrimental in that it slightly decreases

the solids loading possible for a green part, it is beneficial

since smaller primitives result from the increased absorption

of the jetted binder and/or solvent [46].

In addition to enabling 3DP to work with particle sizes

that are typically too small to be spread, spray-drying the

powder eliminates the need for printing a polymeric binder

into the bed as a means of forming primitives (the

traditional 3DP approach, Fig.5a). Instead, the binder used

to form granules can be activated in the powder bed

through the printing of a solvent. The printed solvent will

partially dissolve and deform the granule surface, creating

printed primitives once dried (Fig. 5b). This approach is

preferred not only because the solvents deformation of the

granule surface will bring the ceramic fine particles

together, but also because it is modularthe same solvent

could be used on various ceramic spray-dried granules

(assuming the same polymer coating is used), thus

eliminating the need to reconfigure the material system.

3 Experimental methods

3.1 Material preparation

While a wide variety of transition metal oxides can be

reduced to metal using this procedure (Section 2.1), the

authors have chosen to work with maraging steel in this

work. Maraging steel features high strength and high

fracture toughness and has uniform, predictable shrinkage

during heat treatment. Finally, its constituents are easily

reduced (Eqs.13).

Fe3O44H2! 3Fe4H2O 1

Co3O44H2! 3Co4H2O 2

NiOH2! NiH2O 3

A metal oxide powder system that will chemically convert to

maraging steel (Fe 18.5Ni 8.5Co 5Mo) upon reduction was

Polymer Binder

Metal Oxide Particles

Printed Primitive

Solvent

Spray-dried Granules

Printed Primitive

a

b

Fig. 5 Powder/binder material system options:a coarse particles with

printed binder and b spray-dried fine particles with printed solvent

1400Sintering(1300 C)

1200

1000

800Reduction

(850 C)600

400Temperature(C

)

Binder burnout(450 C)

200

00 5 10 15 20

Time (hr)

Fig. 6 Cycle for reduction and sintering of 3D printed maraging steel

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created by combining iron oxide (Fe3O4), nickel oxide (NiO),

cobalt oxide (Co3O4), and molybdenum metal (Mo) powders

and ball milling them for 24 h. Once mixed, the metal oxide

powder system is spray-dried with a 4 wt.% poly-vinyl

alcohol (PVA) solution (Celvol 203, offered by Celanese

Chemicals). This PVA was chosen because it is water soluble

and is a common binder that works well with almost any

oxide ceramic [47]. The powder particles were spray-dried by

Aero-Instant Spray-Drying Services of Brunswick, Georgia.

3.2 Part fabrication

Once spray-dried, the metal oxide granules are processed

using a ZCorp Z402 three-dimensional printer [48].

Printing tests were performed using two different printed

binding mechanisms: (a) ZCorps standard binder (ZB7

binder) and (b) a mild solvent (an 80/20 mixture of distilled

water and isopropyl alcohol (IPA)). The layer thickness was

held constant across all experiments at 100 m a s i t

provided the highest quality surface upon recoating. Aware

of the porosity of the spray-dried granules, and small

features typical of cellular materials, the saturation level

was set at its maximum value of 2. All other printingparameters are unchanged from default settings.

Once printing has completed, parts are left in the powder

bed for 20 min (often under an infrared lamp) to allow the

binder to fully dry. The green parts are then transferred to a

depowdering station where unbound powder is carefully

removed from the complex cellular geometry using com-

pressed air and a vacuum nozzle in combination.

3.3 Post-processing

Once depowdered, the green parts are reduced and sintered in

an atmosphere-controlled tube furnace in an Ar10% H2environment using the cycle presented in Fig. 6. The

determination of the facets of the sintering scheme were guided

by Cochran and coauthorsearlier experimentation in sintering

metal oxide constituents of maraging steel for the production of

linear cellular alloys [27,28]. The heating cycle is composed

of three phases: debinding of the PVA binder (2C/min ramp

to 450C with 0.5 h hold), reduction of the metal oxide green

part (3C/min ramp to 850C with 6 h hold), and metal

sintering (3C/min ramp to 1,300C with 3 h hold). The holdduration for the reduction and sintering phases were varied via

experimentation; the final values were chosen as they ensured

that the finished parts would have all oxide phases completely

removed and would be as dense as possible.

4 Results

4.1 Phase identification

X-ray diffraction (completed with molybdenum radiation)

was used to identify the phases present in samples created

by the manufacturing process. Count peaks occur at 2=

Table 1 Density measurements for 4 wt.% granule systems

Granule

binder content

Deposited

binder/solvent

Average

relative density

Average open

porosity

4 wt.% Polymeric

binder (ZB7)

59.15% 36.15%

4 wt.% Solvent

(water + IPA)

62.57% 34.4%

Fi 7bFig. 7ba c

b

Fig 7cFig. 7cz

xz

x

yy

Fig. 7 a Schematic of sample

and b micrographs of cross-

sectional surfaces perpendicular

to build direction and c parallel

to build direction

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20.155, 28.651, 35.284, 40.97, 46.067, and 50.755. These

results correspond to a BCC iron phase (JADE phase

identification software; powder diffraction file # 00-0006-

0696) with a figure of merit (FoM) of 3.5. From this, it can

be concluded that no oxide phase is present in the finished

parts; thus the parts are fully reduced.

4.2 Density

As discussed in Section 3.2, two primitive creation

principles were tested: one featuring the printing of a

polymeric binder to bind granules, the other featuring the

printing of a solvent to partially dissolve the spray-dried

granulesbinder. The density and open porosity of multiple

test parts created from both material systems were

calculated using the Archimedes method (assuming marag-

ing steel has a bulk density of 8.2 g/cm3). As can be seen in

Table 1, printing a solvent into the powder bed produces

parts with a higher density and a lower open porosity. The

solvent deforms the spray-dried granules, which in turnbrings the enclosed fine particles closer together and leads

to a better sintering performance and a higher part density.

The relatively low density measurement is attributed to a

poor powder bed density (estimated to between 30% and 40%

in these experiments) and an insufficient deformation of the

spray-dried granules. This is a limitation of the authors

specific embodiment; experiments wherein excess solvent

was applied manually to the powder bed resulted in parts with

an average relative density of 81% and an open porosity of 9%

after reduction and sintering.

4.3 Porosity and shrinkage

In order to further evaluate the quality of the parts created by

this manufacturing process, part porosity is investigated by

examining the cross-sectional surfaces of samples (Fig.7).

Figure7bprovides an opportunity to analyze the porosity

present in a typical cross-sectional layer (xy plane as per

Fig.7a) fabricated by the process when a solvent is used forcreating primitives. It is observed that the pores are aligned

parallel to the direction of the print head travel (y-axis; left to

right in Fig. 7b), and thus are locations in which printed

bands did not successfully overlap orstitchto one another.

This can be attributed to a clogged print nozzle, but is more

likely due to unoptimized binder characteristics (surface

tension, wetting of granules, viscosity, droplet size, etc.) and

associated process parameters (e.g., printed line overlap).

The cross section presented in Fig. 7c provides an

opportunity to analyze the porosity that exists along the

build direction of the part (yzplane as per Fig. 7a; from

the bottom to the top of Fig. 7c). Again, parallel lines ofporosity are observed. These lines correspond to parts

layers and suggest an inadequate level of solvent saturation

into the powder bed. This problem can be alleviated

through an increase in the amount of solvent deposited by

the print head.

Average linear shrinkage, as measured and calculated

(L/Lo) across several printed samples between their

green and sintered states, is 45%. This shrinkage is a

result of both the part porosity caused by unoptimized

printing parameters and the act of reducing metal oxide to

Fig. 8 Sample part featuring

intersecting channels: a CAD

model,b metal oxide green part,

and c part after sintering and

reduction

Fig. 9 Cellular material sample

in its various representations

during the manufacturing pro-

cess chain: a CAD model,

b metal oxide green part, and

c part after sintering and

reduction

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metal (Section2.1). There was slight anisotropic shrinkage

in the parts due to the parallel lines of porosity (Fig. 7); on

average, parts experienced 1% more linear shrinkage in the

direction that is orthogonal to these lines of pores.

4.4 Cellular geometry

Several test parts were created in an effort to characterizethe proposed manufacturing process capability of fabricat-

ing parts of designed mesostructure. Each test part contains

features that are common to geometries typical of cellular

materials: thin walls, small channels, and angled trusses.

& Thin walls: The minimum wall thickness created, as

measured after sintering and reduction, was 270 m. It

should be noted that this is not necessarily the best

possible result of the proposed augmented 3DP process

as printing resolution is primarily dependent on the

printhead of the specific 3DP machine.

&

Small channels: The channel size limit that can becreated is not imposed by the resolution and printing

accuracy of the 3DP process. In its current embodiment,

the limit is imposed by the ability of the green part to

withstand the vigorous depowdering that is necessary to

remove the unbound powder trapped within the chan-

nels. Thus far, channels 2 mm in diameter as printed in

the green state (1.1 mm in diameter after sintering and

reduction) have been successfully created, as seen in

Fig. 8.

& Angled trusses: Angled trusses can be difficult to

process in an AM context as the combination of acute

angles, thin trusses, and relatively large layer thick-nesses can lead to non-overlapping layers. Experiments

have shown that trusses, as small as 1.75 mm in

diameter (green state), can be printed when inclined to

the build plane at angles as low as 20.

With the processs ability to realize the geometrical

building blocks of parts of designed mesostructure verified,

a series of parts featuring complex cellular geometry were

created. An example piece, featuring trusses less than 1 mm

in diameter (post-sintering), is shown in Fig. 9. As can be

seen, the surface roughness is similar to other powder-based

AM processes, which is akin to a sand casting. Additional

parts of designed mesostructure created by this process are

presented in [49].

5 Conclusions

In this paper, the authors present a layer-based additive

manufacturing process for the realization of metal parts

with designed mesostructure. Specifically, metal oxide

green ceramic parts, created by three-dimensional printing,

are sintered and reduced in a hydrogen/argon atmosphere to

chemically convert the part to metal. The green parts are

formed by printing a solvent into a powder bed composed

of spray-dried granules; the solvent deforms the binder-

coated granules, thus pulling the enclosed fine particles

closer together and improving the sintering performance (i.

e., density) of the green part.

The process has been shown to successfully create partswith designed mesostructure (Fig.7). In addition to creating

cellular artifacts, it has fabricated walls as thin as 270 m,

channels as small as 1.1 mm in diameter, and angled trusses

less than 1 mm in diameter. Furthermore, the specific

process embodiment used by the authors has produced

finished parts with an average relative density of 63% and

an average linear shrinkage of 45%.

Many opportunities exist for improving the proposed

manufacturing process. Part density could be improved by

exploring techniques for increasing powder bed density, by

increasing solvent deposition, by optimizing the solvent/

powder bed interface, or by depositing a nanoparticlesuspension into the powder bed. Printing resolution (and

thus, part feature size) could be improved through explor-

ing different print head embodiments. A different recoating

solution might enable the use of finer particles, thus

resulting in smaller printed features and an improved final

part density. As a preliminary cost analysis indicates that

metal parts created by the process cost only ~$3/in3 [49],

the authors believe the process merits further improvement.

Acknowledgments We gratefully acknowledge the funding given

by NSF DMI-0522382. Christopher Williams acknowledges the

financial support provided by the Georgia Tech TechnologicalInnovation: Generating Economic Results (TI:GER) program (NSF

IGERT-0221600). Dr. Michael Middelmas is acknowledged for his

laboratory assistance during the reduction and sintering post-process

(Section3.3). The authors would also like to thank Mr. Joe Pechin of

Aero-Instant Spray Drying Services for his generosity and assistance

in preparing the experimental powder system (Section3.1).

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